Light emitting apparatuses have been developed that have a light emitting device, and a phosphor. The light emitting device emits source light. The phosphor is excited by the source light, and can emit light with a different color from the source light. Thus, the light emitting apparatuses can emit light with various wavelengths based on the additive color mixture principle. For example, in the case where a light emitting device emits primary light with a wavelength in a range of from ultraviolet to a short visible wavelength to excite RGB (Red, Green and Blue) phosphors as wavelength conversion members by using the emitted light, red, blue and green as three primary colors are additively mixed to produce white light.
Light emitting apparatuses have been developed that have a combination of a blue LED (Light Emitting Diode), and phosphors that are excited by the blue light and emits green light and red light. The light emitting apparatuses can emit white light based on the additive color mixture principle. For example, Patent Document 1 discloses a nitride or nitride oxide phosphor with β-Si3N4 crystal structure (β-SIALON) that emits green light. A lighting apparatus has been developed that excites this phosphor and a phosphor of CaSiAlN3:Eu as red luminescent type phosphor by using a blue LED for mixture of light emitted from the LED and the phosphors to emit white light.
In the case where the thus-configured lighting apparatus is used as back light source for liquid crystal display (LCD), color cathode-ray tube (CRT), projection tube (PRT), field emission display (FED), and vacuum fluorescent display (VFD), the NTSC ratio is typically used as an index of natural color tone reproduction on the display. The NTSC ratio refers to a ratio between a reproduction area and the area of a triangle obtained by connecting red, green and blue chromaticity coordinates (x, y) (red (0.670, 0.330), green (0.210, 0.710) and blue (0.140, 0.080)) in the XYZ colorimetric system defined by NTSC (National Television System Committee). Ideally, color reproduction preferably has an NTSC ratio of 100%. In small LCDs for cellular phones, which mainly use white LEDs, brightness is mainly required but color reproduction is comparatively considered less important. However, white LEDs have been used for notebook PCs and large TVs in these days. In this case, color reproduction is highly required.
However, in the lighting apparatus disclosed in Patent Document 1, the β-SIALON phosphor has a peak wavelength of 540 nm, and the CaAlSiN3:Eu phosphor mainly has a peak wavelength of about 650 to 660 nm. Accordingly, it is difficult for the lighting apparatus disclosed in Patent Document 1 to provide color reproduction region that accurately covers the entire range of the NTSC ratio. As a result, there is a problem in that image reproduction cannot be highly accurately provided.
As for light emitting apparatuses that are currently typically used, and includes a blue light emitting device and a phosphor that is excited by the blue light and emits yellow light, the color reproduction (NTSC ratio) of the light emitting apparatuses is about 70%. This value is not enough for the color reproduction requirement. As for a white LED apparatus that has a combination of blue, red and green LEDs, although the white LED apparatus has an NTSC ratio of 100%, it is difficult to provide a white LED apparatus with long life at low cost. The reason is that the drive voltages for the LEDs are different, and deterioration properties of the LEDs are also different. As discussed above, there is an urgent need to develop an LCD back light apparatus that has improved color reproduction (NTSC ratio) required for large size LCDs, and improved brightness, life and cost.
For example, Patent Document 2 discloses a light emitting apparatus that has a combination of a blue light emitting device, and green luminescent type phosphor of (Sr, Ba)2SiO4:Eu and red luminescent type phosphor of CaAlSiN3:Eu that are excited by the blue light and emit green light and red light. The peak wavelength of the (Sr, Ba)2SiO4:Eu phosphor can be varied in a range of 520 to 600 nm. Since a relatively wide color reproduction adjustment range can be provided, it is possible to provide wide color reproduction. However, since the phosphor in this light emitting apparatus greatly deteriorates, there is a problem in that the applicability of this light emitting apparatus is limited.
An example of conventional arts focusing color reproduction nature (NTSC ratio) in LCD can be provided by a liquid crystal display in Patent Document 3. This apparatus as back light source has a spectral peak in a range of 505 to 535 nm. This wavelength is obtained by a green luminescent type phosphor. It is disclosed that this phosphor contains any of europium, tungsten, tin, antimony, and manganese as activation agent. Specifically, MgGa2O4:Mn and Zn2SiO4:Mn are disclosed as the green luminescent type phosphor. However, in the case the aforementioned phosphor is installed together with a light emitting device with a peak wavelength of 430 to 480 nm, the excitation wavelength for this phosphor does not agree with the peak wavelength of the light emitting device. For this reason, the light emission efficiency of the phosphor is very poor. That is, phosphors for practical use level are required not only to have sufficient color reproduction but also to have excellent durability and high light emission efficiency for an excitation light source.
In addition to this, even in the case where phosphors have ideal color reproduction and luminescent properties, if the phosphors are unevenly distributed in an installation area of a light emitting apparatus, the light emission amounts of different colors will be unevenly mixed. This unevenness may cause color unevenness in the mixed light. For this reason, for example, LEDs will have large color tone variation. Consequently, it is difficult to provide LCDs with constant quality. That is, the properties of the entire light emission of a light emitting apparatus largely depend on the mixture state of phosphors.
Also, various types of filters are used as LCD back light filters. The absorptance of a filter varies accordance with light wavelength. For this reason, a light source is required to have a peak wavelength corresponding to the property of a LCD back light filter. However, as for green luminescent type phosphors, a green luminescent type phosphor has not been developed that has excellent properties such as luminescent properties and durability, and has a variable peak wavelength while keeping these properties. For example, although the aforementioned β-SIALON phosphor disclosed in Patent Document 1 has excellent luminescent properties, the β-SIALON phosphor can provide high brightness when emitting light with a peak wavelength of about 540 nm, but its brightness remarkably decreases when its peal wavelength is varied from about 540 nm. Accordingly, the β-SIALON phosphor disclosed in Patent Document 1 is not suitable for a filter that has a filter property corresponding to a peak wavelength of about 530 to 535 nm, which is shorter than 540 nm corresponding to the best peak wavelength of the β-SIALON phosphor. For this reason, a green luminescent type phosphor is required that has a peak wavelength of about 530 to 535 nm, and has excellent properties and the like. Also, a light source is required that has a peak wavelength corresponding to the filter property of an LCD back light filter. Also, a light source is required that can be adjusted to desired NTSC-based color reproduction. To achieve this, as discussed above, each of light sources is required to reduce color unevenness for luminescent property improvement, and wide regional luminescent properties of a combination of the light sources are required to be taken into account. That is, since the applied electric power difference or luminescent property difference among the light sources may cause color variation may occur in surface light emission provided by integrated small light source, a light source is required that can improve color variation. In other words, it is important to adjust the wavelength of a phosphor in accordance with light emission conditions in each light source and a color filter.
As for a combination of an excitation light source and a phosphor, it is required to improve the properties of the phosphor itself such as brightness, color reproduction region and reliability (phosphor deterioration), and additionally to improve the yield (color variation), cost and the like of a product as a light emitting apparatus.    [Patent Document 1] Japanese Patent Publication No. 3921545    [Patent Document 2] Japanese Patent Publication No. 3940162    [Patent Document 3] Japanese Patent Laid-Open Publication No. 2003-121838    [Patent Document 4] Japanese Patent Laid-Open Publication No. 2004-287323